The present invention relates to a flash memory device and, more particularly, to a programming method, in which an interference phenomenon between memory cells can be prevented.
FIG. 1 is a block diagram of a conventional flash memory device.
Referring to FIG. 1, the flash memory device includes a memory cell array 10 and page buffers P1 to Pn (n is an integer). The memory cell array 10 includes a plurality of cell strings Cle to Cne and Clo to Cno. Each of the plurality of cell strings Cle to Cne and Clo to Cno includes a plurality of memory cells (not shown) for storing data. The page buffers P1 to Pn are connected to the plurality of cell strings Cle to Cne and Clo to Cno through an even bit line BLe and an odd bit line BLo. In particular, one page buffer P1 is connected to the cell strings Cle and Clo through the even bit line BLe and the odd bit line BLo, forming a pair.
A general flash memory device operates as follows. The page buffer P1 is described as an example. The page buffer P1 is loaded with data to be programmed, and either the even bit line BLe or the odd bit line BLo is selected by a bit line selection unit (not shown). If the even bit line BLe is selected, a cell selected from cells included in the cell strings Cle connected to the even bit line BLe is programmed.
However, as the level of integration of a flash memory device increases, the line width decreases and the gap between memory cells becomes close. Thus, neighboring cells sharing the same word line are interfered by the voltage of the programmed cell. This phenomenon may occur in all structures of a Single Level Cell (SLC) and a Multi Level Cell (MLC). However, the interference phenomenon between memory cells becomes more profound in the MLC.
Threshold voltage distributions of the MLC are narrower than those of the SLC. In other words, assuming that the range of threshold voltages of the SLC is 1 V, the range of threshold voltages of the MLC is 0.2 to 0.6 V. This is because the MLC has four voltage states. Accordingly, the threshold voltage distribution is inevitably narrowed.
If any one threshold voltage distribution is widened due to the influence of neighboring cells, the threshold voltage distributions may overlap with other states. Furthermore, this phenomenon becomes more profound at the time of an odd page program operation following an even page program operation. This is because cells of a programmed even page are interfered by cells of an odd page that are programmed next. Accordingly, the reliability of devices may be significantly degraded.